Hard disc drives are used in modern computer systems and computer networks to enable users to store and retrieve vast amounts of data in a fast and efficient manner. A typical disc drive houses five to ten magnetic discs which are axially aligned for rotation by a spindle motor at a constant, high speed. An array of read/write heads are controllably positioned adjacent recording surfaces of the discs in order to store and retrieve data from tracks defined on the disc surfaces. The heads fly adjacent the recording surfaces on air bearings established by air currents set up by the rotation of the discs.
The heads are mounted to a rotary actuator assembly to which a coil of a voice coil motor (VCM) is attached. As is known in the art, a VCM includes a pair of magnetic flux paths between which the coil is disposed so that the passage of current through the coil causes magnetic interaction between the coil and the magnetic flux paths, resulting in the controlled rotation of the actuator assembly and the movement of the heads across the disc surfaces.
A closed loop, digital servo circuit is used to control the application of current to the coil, and hence the position of the heads with respect to the tracks. More particularly, the tracks are defined from servo information that is written to the discs during disc drive manufacturing and is stored as a plurality of radially extending wedges on each recording surface. Each servo wedge comprises a plurality of adjacently disposed servo fields, with each servo field defining a different track on the disc surface.
During operation, the servo fields are periodically sampled by a selected head to provide an indication to the servo circuit of the position of the head relative to the disc. When the disc drive is in a track following mode of operation, the servo circuit makes continuous adjustments to the amount of current passing through the coil in order to maintain the selected head over a corresponding track. During a seek operation wherein the selected head is moved from an initial track to a destination track, the servo circuit applies a succession of current values to the coil to first accelerate and then decelerate the head to the destination track in relation to the measured head velocity and a velocity trajectory profile.
A typical disc drive is further provided with data channel circuitry to facilitate the transfer of data between the discs and a host computer in which the disc drive is mounted. As known in the art, the data channel circuitry comprises a write channel which encodes and serializes the user data to be stored so as to generate a succession of write currents which are passed through the selected head to write the data to data fields formatted between adjacent servo fields on the disc. The data channel circuitry further includes a read channel which decodes readback signals generated by the head as the head passes over selected data fields to reconstruct the data and pass the same to the host computer. Error correction codes (ECC) are employed to detect and correct up to a selected number of errors in the recovered data.
Disc drive manufacturers typically produce a large number of nominally identical drives which are individually optimized during the manufacturing process through the setting of parameters that affect the operation of various disc drive circuits, such as the data channel and the servo circuit. Such parameters are well known and typically include write current, write precompensation, servo gain, data and servo level detection thresholds, transversal equalizer tap weights, adaptive filtering parameters and, in disc drives employing magneto-resistive (MR) heads, read bias current and reader offset. Such parameters are used to enable the disc drive to accommodate changes in data transfer rates that occur with respect to the radii on the discs at which the data are stored, noise levels, electrical and mechanical offsets and the like, all of which generally affect the operation of the drive.
Accordingly, the parameters are often set to an initial value during disc drive operation and then optimized against predefined acceptance criteria (for example, measured read error rate). Disc drives are often further provided with the capability of continually monitoring drive performance and adjusting certain parameters adaptively during operation to maintain optimum levels of performance.
The optimization of a selected parameter typically involves selecting a first value for the parameter, writing test data to one or more tracks, reading the data and calculating an error rate for the first value. The process is then repeated with the parameter being sequentially incremented so that a population of error rates is obtained for a range of parameter values. An optimum parameter value is then selected from the range of parameter values, the optimum parameter value providing optimum performance for the drive.
Read error rates in modern drives are relatively low; for example, a typical disc drive might detect one read error out of 10.sup.9 bits read (usually expressed in the form 1.times.10.sup.-9 read errors/bit). Because read error rates are so low, a relatively large amount of data must usually be written and then read in order to facilitate differentiation between the effects of one parameter value to the next. Thus, parameter optimization takes a significant amount of time to complete, in that error rates are typically calculated for a number of different, incremented parameter values over a predetermined range of values. The parameters are further typically optimized on a per-head basis, and per-zone in disc drives employing zone based recording. Parameters are often also optimized for different ambient temperature conditions and for multiple parametric interdependencies (where the effect of one parameter is dependent in part upon the particular value of another parameter).
Because the trend in the industry is to provide ever increasing amounts of parametric adaptivity in successive generations of drives, in many cases there is simply not enough time in the manufacturing cycle to optimize every parameter that could be optimized in a drive. Hence, disc drive manufacturers typically attempt to optimize only those parameters that provide the greatest improvements in disc drive performance in the limited manufacturing cycle time available in which to test each drive. Remaining parameters are often simply set to predefined values which may or may not be later optimized during field operation.
Accordingly, there is a need in the art for an improved approach to selecting parameter values for a disc drive so as to reduce the amount of data that must be written and read in order to differentiate the effects of various parameter value combinations, so that greater levels of optimization can be achieved.